I’m a sucker for this kind of thing, really: Two midsize powercats, on loan to PMY from NauticBlue Power Yacht Vacations, an offshoot of The Moorings, the worldwide charter outfit. A testing venue shimmering with scenic beauty. And finally, an immense and groovy pile of test gear, complete with freshly calibrated fuel-flow-measuring equipment and enough compression-type fittings to stock a diesel-repair shop.
The objective was simple—to scientifically quantify performance differences between two NauticBlue 464s, one with two big, stainless steel hydrofoils, and one without. Uniformity was key, of course. So both boats had to have the same engines—twin 370-hp Yanmar 6LYA-STP diesels—and the same water and fuel levels (full) and be judged under the same sea conditions—one-footers, more or less—on the same stretch of water: Road Harbor, Tortola, British Virgin Islands. Prop diameter and pitch were the only parameters that were out of synch. While my conventional 464 had two 22x31 four-blade wheels, the hydrofoil-equipped model sported oomphier 23x331?2 props, presumably to both generate and harmonize with improved speed and performance.
One other detail that’s worth mentioning up front: The 464 with foils had a semiadjustable feature, meaning that while her forward foil was fixed, her aft one was movable. More specifically, her stern foil could be hydraulically trimmed down from the horizontal by approximately 17 degrees for an optimized angle of attack via a toggle switch at the upper helm station. Before departing for sea trials from NauticBlue’s facility on the shores of Road Harbor, I dove on the hydrofoil 464 with snorkeling gear to examine the stern foil, noting that its ends were engineered to pivot between the boat’s demihulls via a simple trim-tab actuator, a pulley system, and a length of cord. Not fancy perhaps, but simple and effective.
It was a gorgeous day on Road Harbor. While I noted no obvious handling differences between the two test boats in open water, the data I recorded was dramatic and thoroughly validated claims made by the originator of the foil system on the foil-equipped boat, South African mechanical engineer Karl Gunter Hoppe. The boat exceeded the speed of her more conventional sistership by Hoppe’s predicted 40 percent and then went on to trounce the poor thing by a whopping 47 percent. The hydrofoil 464 achieved 31.5 mph at WOT, then upped the ante to 33.3 mph with the foils fine-tuned. The conventional cat did 22.6 mph, tops.
Other findings were also impressive. While the cat without foils was more efficient at lower rpm (quite likely because of lower underwater drag), she was comparatively inefficient on the high end and offered significantly less range. Note that 178 statute miles at 2900 rpm is nearly 67 percent less than the 266 statute miles the hydrofoil boat gets without its aft foil tilted. And it’s darn near 60 percent less once the tilt toggle is deployed!
I struck one minor snag with the foil boat, however. While the cat without foils tended to plane seamlessly, as indicated by the smooth, parabolic ascent of its acceleration curve, the foil boat did some mild struggling between 12 mph and 25 mph, as indicated by the shallow pothole, both in the curve with the stern foil actuated and the one without. Again, this was probably due to the foils’ extra underwater drag.
Then there was another problem. Most likely because my test boats had been topped off with questionable fuel prior to our comparison wring-out, the engines in both failed to turn up to a maximum-rated 3300 rpm. While this detail certainly cut performance, I don’t think it skewed the message of my test results, especially since all four engines turned exactly the same rpm at WOT.
Conclusion? Without a doubt, next time I hit the trail for Tortola to charter a NauticBlue powercat, I’m holding out for super-fast, super-efficient hydrofoils—not just because they can optionally reduce running time, but because they’ll save me a bundle on fuel.
NauticBlue Power Yacht Vacations
Test Boat Specifications
- Test Engine: 2/370-hp Yanmar 6LYA-STP diesel inboards
- Transmission/Ratio: ZF 60IV-2.5/2.49:1
- Props: 22x31 4-blade Nibral for conventional hull; 23x331?2 5-blade Teignbridge Aquafoil for foil-equipped hull
This article originally appeared in the June 2004 issue of Power & Motoryacht magazine.